The invention relates broadly to liquid-liquid extractions. More specifically, the invention is directed to an improved sieve tray column, for performing liquid-liquid extractions.
In a liquid-liquid extraction, also referred to as solvent extraction, the objective is to remove one or more components from a liquid mixture by intimate contact with a second liquid. The second liquid is thus immiscible with the liquid mixture, but it functions as a solvent for removing the components from the mixture. A conventional apparatus widely used for performing liquid-liquid extractions is known as a sieve tray column, sometimes referred to as a perforated plate column.
A typical sieve tray extraction column consists of a vertical cylindrical column which includes a series of spaced apart, perforated trays, which are positioned horizontally from top to bottom in the column. A solid vertical plate is attached to one side of each perforated tray. The vertical plates are positioned on opposite sides of each tray, such that they define a staggered pattern within the column. Each plate extends from the tray to which it is attached to a point above or below the next tray. Each of the vertical plates is also spaced from the inner surface of the column wall, so that a conduit is provided for liquid to flow in a side-to-side path from one tray to the next. This conduit is referred to as an upcomer or a downcomer, depending on flow direction of the solvent liquid in the column. The column also includes an entrance port and an exit port, both at the top and at the bottom of the column. This enables a countercurrent flow of the immiscible liquids within the column.
In a typical procedure for column extraction, the liquid that is divided into droplets is referred to as the dispersed phase liquid, and the other liquid is known as the continuous phase liquid. To give an example of this procedure, assume that the heavier liquid (the more dense liquid) is the dispersed phase. The heavier liquid is passed into the column through the entrance port at the top. The lighter liquid (the less dense liquid) is pumped under pressure into the column through the entrance at the bottom.
As the heavier liquid flows downwardly in the column it passes through the holes in each tray. At the same time the lighter liquid flows upwardly (countercurrent) in the column through the conduit space defined between each of the vertical plates and the column wall. In its downward flow the heavier liquid coalesces at the top surface of each tray and then breaks up into droplets as it flows through the tray openings. The droplets thus form a dispersed phase within the upwardly flowing continuous phase liquid.
The heavier liquid collects at the very bottom of the column, where it forms an interface with the lighter liquid. The liquid-liquid interface is defined just below the entrance port for the lighter liquid. This enables continuous removal of the heavier liquid through an exit port below the interface level. At the same time, the lighter liquid is continuously drawn off through an exit port at the top of the column.
According to conventional practice there are many situations in which it is desirable to disperse the lighter liquid, rather than utilizing it as a continuous phase. This can readily be done by inverting the column. For example, when the column is inverted the lighter liquid becomes the dispersed phase by passing upwardly through the sieve trays. At the same time, the heavier liquid becomes the continuous phase as it flows through the downcomer countercurrent to the dispersed phase.
Before a production size extraction column or tower is built, the usual practice is to obtain design data by duplicating the extraction procedure in a small scale unit. To obtain useful data from the small scale unit, it is essential for certain structural features to be the same in the small scale unit as in the production apparatus. For example, the perforations in the sieve trays of each column must have the same diameter and they must be arranged in a similar pattern on the tray. Another feature which must correspond in each column is the actual area of the upcomer (or downcomer) in relation to the cross sectional area of the column.
As an example, typical engineering specifications call for the sieve tray openings to be about 1/8 to 1/4 inch diameter. The preferred arrangement of the openings is to position them on the corners of triangles or squares, with 1/2 to 3/4 inch spacing. The upcomer (or downcomer) space usually comprises about 10 to 40 per cent of the inside cross sectional area of the column.
The actual area will depend on the ratio of the flow rate of the continuous phase to the flow rate of the dispersed phase. One of the reasons given for these requirements is to prevent flooding of the column by entrainment. A simplified explanation of this problem is that the linear velocity of the continuous phase liquid, as it passes through the upcomer (or downcomer), must not exceed a certain limit. If this limit is exceeded, the dispersed phase liquid will become entrained in the continuous phase liquid and a subsequent recycling of the droplets will flood the column.
In small scale extraction columns it is difficult to maintain all of the engineering specifications which are called for in the larger, commercial-size columns. This problem applies particularly to extraction columns having an inside diameter which is less than about 6 inches. To cite a specific example, in a column with an inside diameter of about 1 inch the area taken up by the upcomer (or downcomer) represents a substantial portion of the total cross-sectional area of the column. In this structure, therefore, there is not enough clearance to space the opening in the tray at least 1/2 to 3/4 of an inch from the upcomer (or downcomer) and the column wall. The resulting wall effects on the liquid are especially important in the smaller diameter columns. The primary reason for this effect is that the droplets are much closer to the walls of the column and the upcomer (or downcomer) in the small columns than in the larger diameter columns.
Another problem is frequently encountered in small diameter columns. For example, in a column with an inside diameter of about 4 inches the ring support (or seal) which contacts the sieve tray with the column wall sometimes requires a ring of 1/2 inch thickness inside the column wall. This leaves a plate of 3 inch diameter, about 56 per cent of the cross-sectional area of the column, in which the tray openings and the upcomer (or downcomer) must be placed. The undesirable result is a severe restriction of the flow capacity of the column, in relation to larger diameter columns. Accordingly, the primary objective is to provide extraction columns of small diameter in which the extraction performance will correspond to that of the larger diameter columns.